The bottles made of polyethylene terephthalate (PET) resins are obtained by biaxially drawing and blow molding the injection molded preforms in a test tube shape, and are used in various fields including beverages, foods, and cosmetics. A variety of decorating methods are used to differentiate the bottle products as merchandise. In many cases, the bottles are decorated with printed shrink films. In addition, bottles themselves are decorated by various methods.
Patent document D1 describes an invention relating to a preform having a double wall structure and to a process for biaxially drawing and blow molding such a preform to mold a colored bottle in which the color is gradated. This preform comprises an inner shell piece in a test tube shape made of a PET resin and an outer shell piece fitted around the inner shell piece. The outer shell piece is colored in a certain color density, and the thickness of the peripheral wall gradually becomes thin over an area starting from the bottom to an upper end of the body. However, the process described in this patent document D1 has low productivity and limited patterns of decoration.
Patent document D2 describes an invention relating to a preform formed by an injection molding device. According to D2, a colored resin layer is sandwiched between an outer surface resin layer and an inner surface resin layer by using a multi-nozzle device. The joined resin layers in a molten state are injected into, and fill a mold cavity to form a preform with the colored resin layer laminated as a middle layer. The document D2 also describes an invention relating to a bottle which is formed by biaxially stretching and blow molding this preform and is decorated with the colored resin layer. The process for injection molding the preform by using the multi-nozzle device to sandwich the colored resin layer between two main resin layers has a higher level of productivity than offered by the process described in the patent document D1.